If it weren't for this plugin, my client would not have been able to build their site in WordPress. They needed extensive search capability for an online catalogue loaded with custom fields and custom taxonomy.
We started with the free version and were able to do quite a bit. We have since upgraded to the pro version, and can do even more.
This plugin has great documentation for both free and pro versions, and the support we get with pro is excellent.
There is a lot of customization you can do via the settings page. For anything not covered there, you can customize with php snippets. The forum boards, the plugin's website, and direct help from the developer (for pro users) explain commonly used php snippets. I have learned a lot using this plugin!
I really appreciate that this plugin works very well for both the coder and non-coder alike.